(1) Field of the Invention
This invention relates to a hydraulic drive circuit for load-handling machines such as hydraulic crawler cranes, shovels and the like.
(2) Description of Prior Art
In a hydraulic load-handling machine, the rated presssure of the drive circuit is determined in consideration of the nature of various jobs performed by the machine. However, in actual operations, there sometimes occurs a necessity for a power greater than the rated pressure, for instance, when extracting an implanted, pile, when pulling a dragline bucket which is caught on a big stone or when getting a machine out of a muddy spot. In such a case, a mechanical load-handling machine can produce an instantaneously increased output (for a few seconds) corresponding to 130% to 150% of nornal output, due to the inertia force of the engine flywheel, but the hydraulic counterpart, the maximum output of which is determined by the relief pressure of the main relief valve is incapable of producing a surge of power exceeding that permitted by the rated pressure and thus is considered to have a smaller winch power as compared with its mechanical counterpart.
In order to increase the output of a hydraulic load-handling machine, it is possible to raise the reduction ratio between the hydraulic motor and the winding drum or to raise the relief pressure of the relief valve of the drive circuit. However, the increase of the reduction ratio, not accompanied by an increase of the pump capacity, results in a lower operating speed, and increases in the pump and engine capacities and the reduction ratio results in a higher production cost, noises and fuel consumption. In a case where the relief pressure of the main relief valve is elevated, it becomes necessary to employ a hydraulic pump, motor and other components with higher pressure resistance, also resulting in increased production costs. If the relief pressure of the main relief valve alone is increased regardless of the capacities of the respective hydraulic components, there invariably arise problems of insufficient service life and durability of the hydraulic components.